1. Field of Invention
The present invention relates to a preparation process of a complex oxide catalyst and the application thereof for gas phase oxidation of light alkenes to unsaturated aldehydes.
2. Description of Related Arts
Acrylic acid and methacrylic acid are two kinds of important organic chemical product, which are currently prepared via two oxidation reactions of corresponding alkenes. For example, firstly the propylene is selectively oxidized to acrolein, and subsequently oxidized to acrylic acid; isobutene or tert-butyl alcohol is selectively oxidized to methacrolein, and subsequently oxidized to methacrylic acid. Whatever producing acrylic acid or methacrylic acid, the development of high performance catalyst for the first step, that is, alkenes oxidation to corresponding unsaturated aldehydes, is one of the core techniques of the whole process.
Till now, the reported catalysts used for selective oxidation of light alkenes to unsaturated aldehydes usually are complex oxide catalysts adopting Mo, Bi, Fe and Co as main active components. A few trace elements are added to the components as promoters to further improve the activity and selectivity of the catalyst. Therefore, the components of the catalyst are very complex, and may be up to 10 types. The complexity of the components provokes a series of problems. The most prominent problem is that the precursor salts of the elements contained in the catalyst have remarkable difference on solubility in water, which will result in uneven distribution of the catalyst components and affect the performance of the catalyst directly, such as activity, selectivity and life span of the catalyst.
Nowadays, the complex oxide catalysts used for selective oxidation of light alkenes to unsaturated aldehydes are often prepared via coprecipitation method, which needs dissolving precursor salts of the catalyst in water. Some compounds of elements, such as Co and Fe, have high solubility, so as to be easily dissolved; some have low solubility, such as the compounds of tungsten, stibium and niobium mentioned in the U.S. Pat. Nos. 5,208,371, 4,816,603 and CN1131059A. In order to dissolve theses compounds of low solubility, huge amount of water has to be used. However, a huge amount of heat has to be needed to dry the solution or slurry containing the compound in the subsequence process, which causes a lot of waste. Besides, for some compounds that can be easily hydrolyzed, such as compounds of Bi, a huge amount of water is unfavorable. In order to solve this problem, the patent JP57-12827 adds some acids, such as HNO3, HCl and so on, during the process of catalyst preparation to make the pH value thereof lower than 7, so that the precursor salts of the catalyst can be dissolved evenly. The U.S. Pat. No. 4,224,193 further lower the pH value of the system to a range of 1 to 5. However, these methods are very complex to operate in industry, the excess acids can corrode the reaction device, and a huge amount of toxic gas NOx is produced during the drying and calcining process of the catalyst preparation. The patent EP0420048 strictly controls the amount of HNO3 added, that is to say greatly reduce the amount of HNO3 that is being used, but this problem still has not been solved.
In order to avoid uneven distribution of the catalyst components due to different solubility of the precursor of the catalyst, U.S. Pat. Nos. 5,138,100, 5,583,086 and CN1486787 change the preparation method of the catalyst by dividing the components into two groups according to the different solubility, separately dissolving the two groups of compounds and mixing the two solutions together. This method can improve the performance of the catalyst, but still can cause the uneven of the catalyst components during the mixing process. Moreover, the preparation process becomes more complex, and is not easy to operate in industry.
Therefore, the above-mentioned patents did not solve the uneven problem of the catalyst component due to the different solubility of the precursor salts.
Furthermore, during the first oxidation reaction of alkene to acrylic acid or methacrylic acid, the conversion and selectivity will greatly affect the whole preparation process. Especially in the process of isobutene to methacrylic acid, the unreacted isobutene in the first oxidation reaction will poison the catalyst of the second oxidation reaction of methacrolein to methacrylic acid. Therefore, in order to retain a long and stable operation, the reaction of propylene to acrolein and isobutene or tert-butyl alcohol to methacrolein must use highly active catalyst or run under high temperature, which will lower the selectivity to the unsaturated aldehydes and reduce the life span of the catalyst. The U.S. Pat. Nos. 4,217,309, 4,250,339, 4,258,217 and 4,267,385 cannot obtain high selectivity to methacrolein under the high conversion of isobutene. Therefore, how to operate the reaction under a milder temperature and retain the high activity, selectivity and stability of catalyst is urgent to be solved.
In order to solve the above-mentioned two problems, one is the uneven catalyst components caused by different solubility of the precursor catalyst which worsen the performance of the catalyst, and the other one is the high conversion, selectivity and stability of the catalyst cannot be obtained simultaneously, the present invention provides a new preparation process of this type of catalyst. A catalyst of high activity and selectivity can be obtained by adding a complexing agent during the dissolving process of the precursor salts, so that the oxidation reaction can be done under a milder condition, the life span of the catalyst is prolonged, and the preparation process is easy to operate so as to be suitable to mass production.